05000390/LER-2008-003
| Watts Bar Nuclear Plant | |
| Event date: | 08-07-2008 |
|---|---|
| Report date: | 10-06-2008 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| 3902008003R00 - NRC Website | |
I. PLANT CONDITIONS
On March 24 and 25, 2008, Watts Bar Nuclear Plant (WBN) Unit 1 was starting up after a refueling outage.
WBN entered Mode 2 on March 24, and Mode 1 on March 25. All systems were functioning normally.
II. DESCRIPTION OF EVENT
A. Event
On August 7, 2008, the NRC issued their 2008 2nd Quarter Inspection Report (05000390-08-03), which identified in a noncited violation (NCV) that TVA started WBN Unit 1 since initial plant startup without an operable channel of auxiliary feedwater (AFW) automatic start on a trip of all main feedwater (MFW) pumps as required by Technical Specification (TS) 3.3.2 Function 6.e. The NCV finding was determined to be of very low safety significance because the finding did not represent an actual loss of safety function of a single train for greater than its TS allowed outage time since other initiation signals were available to automatically start the auxiliary feedwater pumps if needed.
With this inspection report, NRC clarified that the instrumentation channels must not only be capable of transmitting a trip signal, but must also reflect the actual operating condition of the main feedwater pumps.
This event is addressed in TVA's Corrective Action Program as Problem Evaluation Report (PER) 147351.
B. Inoperable Structures, Components, or Systems that Contributed to the Event There were no inoperable structures, components, or systems that contributed to the event. However, one of the AFW autostart on MFW pump trip instrument channels was placed in bypass, which indicated that the MFW pump was running, and thus did not reflect the actual condition of the pump.
C. Dates and Approximate Times of Major Occurrences Date� Event � March 24-25, 2008 Entered Modes 2 and 1 while restarting unit from Refueling Outage 8 using two motor-driven AFW pumps, with auto-initiation of AFW on trip of MFW bypassed.
May 25, 2008�Restarted unit from forced outage with auto-initiation of AFW on trip of MFW bypassed.
August 7, 2008�NRC issued 2008 Second Quarter Inspection Report stating that AFW auto-initiation on loss of MFW instrumentation configuration used during past restarts is a TS 3.0.4 violation.
II.ADESCRIPTION OF EVENT (continued):
D. Other Systems or Secondary Functions Affected
None
E. Method of Discovery
NRC identified the violation in inspection report 05000390-08-03.
F. Operator Actions
Operators manually monitored plant status and placed the Auxiliary Feedwater System autostart back into service once turbine driven Main Feedwater Pumps were put into operation.
G. Safety System Responses
All safety systems responded as designed.
III.ACAUSE OF EVENT Plant-specific design was apparently not considered in adoption of Improved Standard Technical Specifications. Plant design was to use the standby main feedwater pump for startup. It was noted in a 2006 noncited violation that use of the standby main feedwater pump violated TS 3.3.2, Function 6.e. Resolution of that finding was to do a 3.0.4.b risk evaluation when starting with the standby pump. NRC inspectors noted that, according to WA procedure, the risk evaluation was not intended to be used on a routine basis, so WA decided to start up using the auxiliary feedwater pumps. It was considered preferable to use the motor-driven pumps without the steam-driven AFW pump (Terry Turbine), so one channel of the autostart instrumentation had to indicate that a MFW pump was running. In fact, the channel accurately reflected the measured parameter, that there was control oil pressure, but the MFW pump was not running. This configuration would ensure that as soon as the MFW pump was started, the AFW autostart circuitry would be functional. WA interpreted that the instrument channel was functional because it transmitted the signal that oil pressure was up. The NRC violation clarified that the channel should not be considered operable when the pump was not running IV.AANALYSIS OF THE EVENT This event was a literal noncompliance with the WBN TS, but had only very minor safety significance. TVA had configured the autostart circuitry so that the AFW autostart would be available as soon as the MFW pump was pumping forward. Prior to that point, AFW was provided to the steam generators. However, only the two motor-driven AFW pumps were being used. The actual autostart signal would start all three AFW pumps.
With the autostart signal unavailable, and less than all the equipment that would be started by that signal, WA was not in compliance with the WBN TS requirement.
The autostart of AFW on trip of all MFW pumps is an anticipatory function. The actual safety function relied upon in the WBN accident analysis is the AFW start on Low-Low Steam Generator Level, which was unaffected by this event.
V.�ASSESSMENT OF SAFETY CONSEQUENCES As noted in the NRC inspection report, the finding was determined to be of very low safety significance because the finding did not represent an actual loss of safety function of a single train for greater than its TS allowed outage time since other initiation signals were available to automatically start the auxiliary feedwater pumps if needed. Additionally, the system was configured so that AFW autostart on trip of all MFW pumps would be available as soon as the MFW pump was pumping forward, which satisfies the intended function.
VI.�CORRECTIVE ACTIONS A. Immediate Corrective Actions As an interim measure, since NRC identified this noncompliance, WA has modified its startup process to enter Mode 2 with both channels indicating that the MFW pumps are tripped and with all three AFW pumps running to feed the Steam Generators. WA then starts one of the MFW pumps and resets the instrument channel, which will support entry into Mode 1, to a level at which the second feedwater pump can be started.
B. Corrective Actions to Prevent Recurrence WA has submitted a License Amendment Request, TS-08-07, to change applicability of this requirement so that it will only apply once the MFW pump is pumping forward. This will permit operation as described in the noncited violation.
VII.�ADDITIONAL INFORMATION
A. Failed Components
None.
B. Previous LERs on Similar Events to start the plant with the standby MFW Pump. That event was identified in NRC Inspection Report 2006-004.
C. Additional Information:
None.
D. Safety System Functional Failure There was no safety system functional failure.
E. Loss of Normal Heat Removal Consideration There was no loss of normal heat removal capability.
VIII. COMMITMENTS
There are no regulatory commitments in this submittal.